The present invention relates in general to a telephone system and in particular to a method of and device for determining and evaluating current conditions in a two-wire loop of a telephone circuit including at least one subscriber's station and an exchange station, whereby two sources of constant current are connected to the loop or circuit symmetrically with respect to the subscriber's station so as to keep a constant current in the circuit irrespective of the length of the line. The method and circuit of this invention are of the type in which current through the telephone wires is measured by means of a measuring device, whereby in the case when different current values are measured, a regulating device is connected to the separate sources of constant current to apply to the latter different reference voltages to supply in the telephone wires a maximum limit current at which the regulating device is closed and means for placing a call to the subscriber's station are activated in a central control device.
From the German published patent application No. 2,556,157 a circuit arrangement for supplying current to a subscriber's apparatus in telephone systems in accordance with the aforedescribed method is described. In this known arrangement, the subscriber's circuit, including the subscriber's terminal such as a telephone apparatus, current is supplied through the telephone line as soon as the user by picking up the receiver closes a corresponding cradle contact. The current supply device which includes two current generators (constant-current sources) arranged symmetrically to the wires leading to the subscriber's station, generates a constant supply current which is independent of the length of operation and on the applied power. The subscriber's stations are provided with a ground button on each telephone apparatus. When the user activates the ground button, one of the two telephone cables of the corresponding subscriber's circuit or loop is grounded. In order to prevent the occurrence of an excessive current through the loop when the latter is short-circuited to ground, and which might cause overheating of the current supplying device, there is provided a zener diode connected to the common connection point of two resistors (measuring device) connected symmetrically to the loop. For evaluating the current conditions in the loop and for detecting the actuation of the ground button, transistors are provided in the loop of the subscriber's circuit, and after picking up the receiver the transistors produce a voltage which can be detected on a central control equipment.
From the German published patent application No. 2,925,409 a circuit arrangement is known by means of which the reporting of a called subscriber of the telephone system can be recognized. Through the detection of zero passages of the speech current passing through the telephone line control clock pulses are generated which are applied to a shift register for shifting at the time points of their occurrence a signal which has been applied to the input of the shift register from the so-called loop current indicators and defining the reporting of the subscriber. In order to distinguish the subscriber's answer during the pause of ringing and during the generation of the ringing current, the output signal of the shift register, which results after several zero passages of the ringing current, is applied together with the output signal from the loop current indicator to an AND-gate.
Both of these prior-art embodiments of cicuit arrangements perform an evaluation of current flowing through the lines of the telephone loop. In evaluating direct and alternating currents flowing during the transmission of ringing signals and by processing these currents by transistors or optoelectric couplers, the latter are prone to damage by excessive voltages occurring in the subscriber's circuit as well as in the evaluation circuit and in the additional circuits applied to the loop. Moreover, the prior-art circuit arrangements of this kind have the disadvantage that pulse-like interference generated in the subscriber's circuit may cause errors in digital evaluation of the loop currents.